Like ssl3_get_client_key_exchange, it is split into three parts:
- If PSK, query the PSK and write out the PSK identity.
- Compute the base pre-master secret.
- If PSK, compute the final pre-master secret.
This also fixes some double-frees on malloc failures in the ECDHE case. And it
avoids using the handshake output buffer to start the premaster secret.
Change-Id: I8631ee33c1e9c19604b3dcce2c676c83893c308d
Reviewed-on: https://boringssl-review.googlesource.com/2062
Reviewed-by: Adam Langley <agl@google.com>
93d67d36c5